Tiny robots no bigger than a cell could be mass-produced and used to monitor conditions inside an oil or gas pipeline. The key to making such tiny devices in large quantities lies in a method the team developed for controlling the natural fracturing process of atomically-thin, brittle materials, directing the fracture lines so they produce miniscule pockets of a predictable size and shape. Embedded inside these pockets are electronic circuits and materials that can collect, record, and output data.

The process, called “autoperforation,” is described in a paper by MIT Professor Michael Strano, postdoc Pingwei Liu, graduate student Albert Liu, and eight others at MIT.

The system uses a two-dimensional form of carbon called graphene, which forms the outer structure of the microscopic devices, called “syncells” (short for synthetic cells). One layer of the material is laid down on a surface, then tiny dots of a polymer material, containing the electronics for the devices, are deposited by a sophisticated laboratory version of an inkjet printer. Then, a second layer of graphene is laid on top.

People think of graphene, an ultrathin but extremely strong material, as being “floppy,” but it is actually brittle, Strano said. But rather than considering that brittleness a problem, the team figured out that it could be used to their advantage.

“We discovered that you can use the brittleness,” said Strano, who is the Carbon P. Dubbs Professor of Chemical Engineering at MIT. “It’s counterintuitive. Before this work, if you told me you could fracture a material to control its shape at the nanoscale, I would have been incredulous.”

Controlled fracture

But the new system does just that. It controls the fracturing process so rather than generating random shards of material, like the remains of a broken window, it produces pieces of uniform shape and size. “What we discovered is that you can impose a strain field to cause the fracture to be guided, and you can use that for controlled fabrication,” Strano said.

Courtesy: Steve Rourke, CFE Media

When the top layer of graphene is placed over the array of polymer dots, which form round pillar shapes, the places where the graphene drapes over the round edges of the pillars form lines of high strain in the material.

“Imagine a tablecloth falling slowly down onto the surface of a circular table,” Albert Liu said. “One can very easily visualize the developing circular strain toward the table edges, and that’s very much analogous to what happens when a flat sheet of graphene folds around these printed polymer pillars.”

As a result, the fractures are concentrated right along those boundaries, Strano said. “And then something pretty amazing happens: The graphene will completely fracture, but the fracture will be guided around the periphery of the pillar.” The result is a neat, round piece of graphene that looks as if it had been cleanly cut out by a microscopic hole punch.

Because there are two layers of graphene, above and below the polymer pillars, the two resulting disks adhere at their edges to form something like a tiny pita bread pocket, with the polymer sealed inside. “And the advantage here is that this is essentially a single step,” in contrast to many complex clean-room steps needed by other processes to try to make microscopic robotic devices, Strano said.

The researchers have also shown that other two-dimensional materials in addition to graphene, such as molybdenum disulfide and hexagonal boronitride, work just as well.

Faux cell

Ranging in size from that of a human red blood cell, about 10 micrometers across, up to about 10 times that size, these tiny objects “start to look and behave like a living biological cell. In fact, under a microscope, you could probably convince most people that it is a cell,” Strano said.

This work follows up on earlier research by Strano and his students on developing syncells that could gather information about the chemistry or other properties of their surroundings using sensors on their surface, and store the information for later retrieval, for example injecting a swarm of such particles in one end of a pipeline and retrieving them at the other to gain data about conditions inside it. While the new syncells do not yet have as many capabilities as the earlier ones, those were assembled individually, whereas this work demonstrates a way of easily mass-producing such devices.

Apart from the syncells’ potential uses, the way the tiny devices are made is itself an innovation with great potential, Albert Liu said.

“This general procedure of using controlled fracture as a production method can be extended across many length scales,” he said. “[It could potentially be used with] essentially any 2-D materials of choice, in principle allowing future researchers to tailor these atomically thin surfaces into any desired shape or form for applications in other disciplines.”

This is “one of the only ways available right now to produce stand-alone integrated microelectronics on a large scale” that can function as independent, free-floating devices, Albert Liu said. Depending on the nature of the electronics inside, the devices could be provided with capabilities for movement, detection of various chemicals or other parameters, and memory storage.

Gregory Hale is the editor and founder of Industrial Safety and Security Source (ISSSource.com), a news and information website covering safety and security issues in the manufacturing automation sector. This content originally appeared on ISSSource.com. ISSSource is a CFE Media content partner. Edited by Chris Vavra, production editor, CFE Media, cvavra@cfemedia.com.